The present invention relates generally to implantable medical devices and more particularly to implantable medical devices intended for use in monitoring a patient""s heart rhythm.
Implantable pacemakers and cardioverters monitor the heart""s rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators which have the capability of storing information related to monitor heart rhythms include U.S. Pat. No. 4,223,678 issued to Langer et al., U.S. Pat. No. 5,722,999 issued to Snell, U.S. Pat. No. 5,513,645 issued to Jacobsen et al. and U.S. Pat. No. 5,312,446 issued to Holschbach et al. In addition, there have recently been developed implantable monitoring devices that do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient""s heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued to Adams and U.S. Pat. No. 5,497,780 issued to Zehender.
In conjunction with implantable devices as described above, information stored relating to a patient""s heart rhythm may include information relating to heart rate trends over time, as disclosed in U.S. Pat. No. 5,088,488 issued to Markowitz et al., U.S. Pat. No. 5,330,513 issued to Nichols et al. and U.S. Pat. No. 5,603,331 issued to Heemels et al. as well as information relating to heart rate variability over time, as disclosed in U.S. Pat. No. 5,749,900 issued to Schroeppel et al., U.S. Pat. No.5,466,245 issued to Spinelli et al., U.S. Pat. No. 5,411,131 issued to Yomtov et al. and U.S. Pat. No. 5,437,285 issued to Verrier et al. Typically, measurements of heart rate trend in such devices are accomplished by continually measuring heart rate over a defined time period, and calculating average heart rates for successive shorter time periods within the defined time period for later telemetry to an external device. Gradual increases in average heart rate over extended time periods are known to be an indicator of decompensation, a phenomenon that takes place during the progression of clinical heart failure.
The present invention is directed toward an implantable device having enhanced capabilities for monitoring a patient""s heart rate trends over extended periods of time. The information collected by the implantable device is stored and telemetered to an associated external device such as a device programmer for display and analysis. Heart rates are measured by measuring the time intervals between sensed depolarizations of a chamber of the patient""s heart and preceding sensed depolarizations or delivered pacing pulses. Intervals may be measured in the ventricle and/or atrium of the patient""s heart. The measured intervals are referred to hereafter as xe2x80x9cheart intervalsxe2x80x9d. The measured heart intervals during defined time periods are used to calculate average heart rates or average heart intervals associated with the time periods. Preferably the average heart rate takes the form of a mean heart rate, but in some embodiments, the median heart rate over the time periods may be employed or the most common heart rate or interval based on a in a histogram of measured heart intervals or other equivalent value may be substituted. For purposes of the present application, the term xe2x80x9caverage heart ratexe2x80x9d should be understood to include mean, median or other equivalent values indicative of the general heart rate or heart interval.
Rather than simply measuring average heart rate values over successive time periods, the implantable device instead measures successive average values of heart rates measured during discontinuous time periods, preferably chosen to occur during times of particular interest, for example during defined time periods during the night and/or day. Preferably the measurements are taken and stored over a period of weeks or months. In a first embodiment, measurements are during the night during a period of time in which the patient is likely to be sleeping. In this context, measurement of the trend of night heart rates taken, for example over the period of time between 12:00 a.m. and 4:00 a.m . is believed to be particularly valuable. Night heart rate is predominantly controlled by the parasympathetic nervous system, and progression of heart failure is usually associated with abnormal excitation of the sympathetic nervous system, leading to increases in night heart rate.
In addition, long-term trends of daytime heart rates may also be collected, for example over periods of time between 8:00 a.m. and 8:00 p.m. Daytime heart rate is primarily controlled by the sympathetic nervous system and thus differences in day and night heart rates can be used as a measure of autonomic dysfunction and have been shown to be different in heart failure patients when compared to age matched individuals with normal hearts. In the context of an implantable pacemaker, comparisons of trends of day and night heart rates to the lower or base pacing rate of the pacemaker may also provide useful physiological information. This comparison may be especially valuable in pacemakers which store information regarding trends of physiologic sensor outputs or regarding trends of pacing rates based upon physiologic sensor outputs as in U.S. patent application Ser. No. 09/078,221, filed May 13, 1998 by Stone et al, incorporated herein by reference in its entirety.
In a preferred embodiment of the invention, the implantable device includes a sensor indicative of exercise level either measured directly using a physiologic sensor such as an accelerometer or piezo-electric sensor or measured indirectly by means of a sensor of metabolic demand such as a pressure sensor, oxygen saturation sensor, stroke volume sensor or respiration sensor. In this embodiment of the invention, measurements of heart rhythms are made only in response to the sensor""s determination that the patient is at rest, in order to produce a long-term trends of resting heart rates during the defined time intervals. Even over relatively long time frames, a patient""s level of activity may vary substantially, and changes in average heart rates can be masked by such variations in exercise level. By limiting the measurements of heart rates to times during which the patient is known to be at rest, a more accurate indication of the true long-term progression of heart rates can be obtained. In such embodiments the implantable device may collect heart rate information continuously during longer time periods, typically extending at least over several hours. During the longer time periods the device may define a series of shorter time periods, typically extending over several minutes, and will employ heart rate information collected during a preceding one of the shorter time periods only if the sensor indicates the patient was at rest during the shorter time period.
In some preferred embodiments, particularly those intended for use in patients known to suffer from tachyarrhythmias, the implantable device is also configured to reject intervals between depolarizations associated with tachyarrhythmias. In such embodiments the implantable device may define a minimum cumulative duration of non-rejected heart intervals as a prerequisite to calculation of an average rate value for a defined time period.
In devices employing physiologic sensors, the device may correspondingly also store values indicative of the general levels of sensor output during daytime and nighttime periods may also be collected. In such embodiments, average sensor output values, including the various types of averages discussed above in conjunction with calculation of average heart rates may be employed. Alternatively, a sum or total of all generated sensor outputs during relevant time periods may be employed.